The present invention relates in general to a method and apparatus for planning a pressure survey, and in particular, a method and apparatus for using a pressure survey tool so that the user may predict in advance the number of pressure tests needed to be performed in a pressure survey such that the results fall within a desired standard deviation of the pressure gradient. The present invention further relates in general to a method and apparatus for performing a pressure survey, and in particular, a process of measuring pressure gradient and standard deviation thereof for downhole fluid bearing formations using information collected from pressure survey apparatus such that the requisite amount of information is collected to ensure the measurements actually received are within the desired standard deviation.
The pressure of an earth formation and its pressure gradient are important characteristics to determine for a subsurface geological formation since, among other things, such information is utilized in reservoir calculations to determine the oil, gas, and water in place in the formation. The more accurate these calculations, the better exploitation can be made respecting both economical and technological recovery of the hydrocarbon fluids.
Currently, pressure of an earth formation is measured from within a drilled borehole using pressure survey tools such as, for example, Schlumberger""s MDT (Modular Dynamic Tester) and Baker-Atlas"" RCI (Reservoir Characterization Instrument). Schlumberger""s MDT tool is generally described in U.S. Pat. No. 4,860,581 to Zimmerman et al., and is also further generally described in U.S. Pat. No. 5,969,241 to Auzerias, which patents are herein incorporated by reference.
A xe2x80x9cpressure testxe2x80x9d is the formation pressure measured at a given depth by a sonde (such as an MDT or RCI) designed to make contact with the formation and place a pressure transducer in contact with it. A xe2x80x9cpressure surveyxe2x80x9d is a series of formation pressure tests taken at different depths of a formation (geologic zone) of interest.
To use an MDT or RCI tool, during a pressure test, the tool is lowered into a borehole, and a packer of the tool is placed against a portion of the borehole wall to isolate that portion of the formation from borehole fluids. A probe, which is part of the tool, is surrounded by the packer. The mudcake around the localized area is usually cleaned. The pressure applied at the probe is drawn down whereby the pressure at the isolated portion of the borehole wall decreases to a pressure well below that of the formation to be tested. This reduced pressure further cleans the isolated portion of the borehole wall formation (i.e., further cleaning the mudcake in a localized area). The probe then is filled with formation fluid, during the applied draw-down. The pressure gauge connected to the chamber then indicates pressure of the earth formation (the xe2x80x9cformation pressurexe2x80x9d). This procedure is repeated at different depths within the formation to perform the pressure survey.
FIG. 1 generally depicts a graph illustrating the change in pressure recorded by a pressure transducer over time, during a single pressure test.
In FIG. 1, the y-axis 101 indicates the pressure recorded by a pressure transducer and the x-axis 102 indicates the time during which the pressures are recorded. Prior to drawdown, the borehole pressure 103 is recorded. During drawdown 104, the pressure recorded reflects that the pressure has been reduced between the borehole wall and the transducer. The pressure is then allowed to build-up during the buildup period 105 until it reaches equilibrium. This equilibrium pressure is the formation pressure 106.
A pressure survey tool is characterized by several accuracy specifications which define its capabilities. These are usually warranted by the manufacturer in some fashion, and are verifiable by testing. Such testing is sometimes referred to as xe2x80x9ccalibration data,xe2x80x9d which calibration data testing, normally determined in a laboratory, reflects the standard performance data of the pressure transducer used to perform the pressure tests.
The accuracy specifications normally given for a pressure transducer used in the MDT and RCI tools are full scale accuracy, linearity, repeatability, and resolution. The pressure transducer is expected to work within these specifications whenever it indicates an equilibrium (stable) pressure value, such as formation pressure.
Repeatability is the specification of a pressure transducer tool associated with determining the accuracy of the gradient. Repeatability is defined as the maximum deviation between all successive measurements of the same quantity, when each of the measurements is approached from the same direction. This is the appropriate accuracy specification to apply to the analysis of the gradient, because each pressure measurement used to calculate the gradient is taken in the same direction. The gradient of a formation is the linear change of pressure as depth changes (typically expressed in psi/ft).
When a specification of repeatability is stated, usually in terms of xc2x1X PSI, it means that all repeatability tests must yield values equal to or less than X. For example, if a transducer records a pressure of 4352.05 PSI when approached from below (as is shown in FIG. 1), and if its repeatability specification is xc2x10.5 PSI, all subsequent readings of the same pressure must read between 4351.55 PSI and 4352.55 PSI, if the pressure is approached from below. This establishes an error band for repeatability, within which all such measurements must fall.
In quality control work such an error band (one within which all deviations must fall) is established by the 3"sgr" rule as reflected in Lawrence S. Aft, Fundamentals of Industrial Quality Control, Addison-Wesley Publishing Company, 1986. The 3"sgr" rule establishes an error band within which theoretically 99.7% of all errors caused by random deviation will fall. In practical terms, if all pressure transducers of a certain type have repeatabilities which fall in an error band of xc2x1X PSI, then the standard deviation of the sample of all such pressure transducers is equal to X/3 PSI.
Values of pressure gradient and standard deviation of pressure gradient are needed to evaluate the nature of formation fluids (gas, oil, water). The more accurate and traceable these values are, the better the results. Pressure surveys are very expensive. The user gains valuable knowledge if the user can plan the survey in advance, and do just as many pressure tests as are required to assure the user that its accuracy needs are met.
While pressure gradient and standard deviation of pressure gradient can presently be calculated by various known methods, it is believed there is no known method for planning a pressure survey based on desired standard deviation of gradient. Thus, there is a need for a method and apparatus to plan a pressure survey so that a pre-desired standard deviation of pressure gradient will be accurately determined beforehand.
Furthermore, there is a need to plan a pressure survey, so the user can know in advance what the standard deviation of pressure gradient will be. This information would also allow the user to increase or scale back the equipment used, so as to achieve a desired standard deviation of pressure gradient in a proper economical and technological fashion.
Furthermore, there is a need to be able to evaluate the pressure tests of the pressure survey as each pressure test is completed to determine when the pre-desired standard deviation of pressure gradient is obtained.
A method and apparatus for planning a pressure survey based on desired standard deviation of gradient has been discovered. The present invention provides values of standard deviation of pressure gradient based upon the repeatability of the pressure survey tool used to measure the pressure gradient, which calibration data is independent of the depth of the formation. Once the formation thickness is determined, the number of pressure tests to be performed in a pressure survey can be planned based upon the repeatability of the pressure survey tool to be used and the standard deviation of pressure gradient desired. Based upon this information, the user may adjust the number of pressure tests required by varying the pressure transducer to be used or the standard deviation of the pressure gradient desired for economical and technological reasons.
Furthermore, a method and apparatus for real time comparison between the standard deviation of the pressure gradient based upon the repeatability of the pressure survey tool used with the calculated standard deviation of the pressure gradient of the information recorded by the pressure survey tool during each test. As each pressure test is completed, this comparison can be updated to confirm when sufficient number of pressure tests have been performed to complete the pressure survey with results yielding a standard deviation of the pressure gradient within the parameters pre-desired.